Electroluminescent system

ABSTRACT

An electroluminescent system, includes a light emitting substance which emits light when an electric voltage is applied thereto and which contains (a) inorganic particles which are clustered to provide inorganic clusters; and (b) organic ligand envelopes which are composed of an organic ligand material and which each envelope an inorganic cluster whereby the inorganic clusters are stabilized. The light emitting substance may additionally contain organic molecule chains which connect organic ligand envelopes to one another. Thus, the inorganic clusters are spaced apart by a spacing which may be preselected by providing organic molecule chains having a preselected number of atoms in the organic molecule chains.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electroluminescent system having a substancewhich is light-emitting when an electric voltage is applied.

2. Description of the Related Art

Electroluminescent systems are known. In these, either inorganic ororganic substances are used which can be excited by means of anelectrical voltage to send out light beams. As inorganic substances, forexample, monocrystalline III-V or II-IV semiconductors and powdered ZnScompounds are used which may be doped differently. The drawback in theseelectroluminescent, inorganic systems is that high alternating voltagesmust be available as operating voltage. If these electroluminescent,inorganic systems are intended to be used, for example, in motorvehicles for the lighting of display instruments, the battery chargewhich is present in the motor vehicle as direct voltage must beconverted accordingly. On the one hand, this voltage conversion is verycomplex and, because of the operation at high alternating voltages, italso requires an additional electromagnetic compatibility protection forfurther electronic systems of the motor vehicle.

Furthermore it is known to use electroluminescent, organic substances,for example, polymers, dyes, polymers doped with molecules and polymerblends. Here, it is a drawback, however, that the organic substanceshave an insufficient long-time stability, particularly a thermal and/orchemical stability. This means that the use of electroluminescentsystems with organic substances is not possible in practicalapplications, for example, for the lighting of display instruments inmotor vehicles.

SUMMARY OF THE INVENTION

In contrast, the electroluminescent system having a substance which islight-emitting when an electric voltage is applied, characterized inthat the light-emitting substance (12) is formed by inorganic particles(22) which are connected with one another via organic spacers (26),offers an advantage that an electroluminescent system, which can beproduced in a cost-advantageous manner and which is versatile in itsapplications, can be accomplished in a simple manner. Since thelight-emitting substance is formed by inorganic particles, which areconnected with one another via organic spacers, it is possible in anadvantageous manner to obtain a light-emitting substance wherein thedesired electrical, electronic, chemical, and mechanical properties canbe achieved in a purposeful way. By means of the inorganic particles,which are connected with one another via the organic spacers, a binarysystem is created wherein the inorganic particles take over the electronconductivity of the light-emitting substance, while the organic spacerstake over the mechanical stabilization of the light-emitting substanceand the generation of the light radiation. Therewith, the properties ofthe inorganic particles are ideally combined with those of the organicspacers so that, by way of optimizing the inorganic particles as well asthe organic spacers to fit their specific application, light-emittingsubstances can be accomplished that have a high thermal and/or chemicalstability and that are provided with high quantum efficiency duringelectroluminescence. By means of a targeted installation of functionalgroups in the organic spacers it is possible to purposefully influencethe probability for a radiant recombination of electrons and holes sothat a high luminous efficiency can be accomplished with thelight-emitting substance according to the invention.

An advantageous embodiment of the invention provides that the organicspacers are formed by an organic, e.g., sphere preferably an organicligand, e.g., sphere. The organic ligand envelope determines thepositioning of the inorganic particles within the light-emittingsubstance because the ligand envelopes enter into a chemical combinationwith one another which determines the mechanical stability of thesubstance and thus the position of the inorganic particles. By selectinga thickness for the ligand envelope with which the inorganic particlesare surrounded, the spacing of the inorganic particles with respect toone another can be set. By selecting the spacing between the inorganicparticles, a so-called tunneling of electrons can be set by way of theligand envelope. Preferably, the thickness of the ligand envelope isselected such that the electron conduction through the light-emittingsubstance is ensured by means of the inorganic particles, with theelectrons tunnelling through the ligand envelope. The ligand envelopemade of organic substances has a high mobility for holes so that thesecan move within the ligand envelope. This resulting tunnel conductivityensures the current flow through the inorganic particles, even thoughthese are spaced apart with respect to one another due to the organicsheath that surrounds them.

A further advantageous embodiment of the invention provides that theorganic ligand envelopes around the inorganic particles are connectedwith one another by means of organic molecule chains. This results in across linkage of the inorganic particles within the light-emittingsubstance, with it being possible to set the spacing between theinorganic particles by a number of atoms of the respective moleculechains. In this manner, the tunnel conductivity between the inorganicparticles can be set in a very advantageous manner by way of the numberof atoms within the molecule chains, that is, whether the distancebetween two adjacent inorganic particles is larger or smaller.

Further advantageous embodiments of the invention ensue from theremaining features which are cited in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in greater detail inembodiments by way of the associated drawings. These show:

FIG. 1 a schematic sectional representation through anelectroluminescent system in a first embodiment variant;

FIG. 2 a schematic sectional representation through anelectroluminescent system in a second embodiment variant, and

FIG. 3 examples of organic molecule chains that are used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an electroluminescent system which is generallyidentified by 10. The illustration schematically shows a detail of asection through a system 10. The system 10 has a light-emittingsubstance 12 which is arranged between a first electrode 14 and a secondelectrode 16. The electrodes 14 and 16 are embodied to be planar so thatthe light-emitting substances 12 disposed between the electrodes 14 and16 are also embodied to be planar between the electrodes 14 and 16. Theelectrodes 14 and 16 are connected to a voltage source 18 which may be,for example, the battery of a motor vehicle. Here, a first electrode 14is connected to ground and a second electrode 16 to the positive pole.The layer structure formed by the electrodes 14 and 16 and the substance12 is arranged on a carrier 20.

The light-emitting substance 12 is comprised of inorganic particles 22,each being surrounded by an organic ligand envelope (sphere) 24. By wayof the considerably simplified illustration it becomes clear that eachof the inorganic particles is provided with a ligand envelope 24.Therewith it is ensured that the inorganic particles 22 cannot contactone another directly. The ligand envelopes 24 thus form a spacer 26between the inorganic particles 22. Depending on the thickness of theligand envelopes 24, the spacing between two adjacent inorganicparticles 22 has a larger or smaller size. Usually, the ligand envelopes24 are organic compounds with a thickness of a few atomic positions.

The inorganic particles 22 may be formed, for example, by electricallyconductive clusters. The clusters may be semiconductor clusters i.e.,clusters containing atoms of semiconducting materials, (for example,CdTe, CdSe, . . . ) or metal clusters i.e., clusters containing metalatoms, (for example, gold, palladium, platinum, nickel, . . . ). Theclusters or the inorganic particles are present in a grain size in thenano range. The organic ligand envelope 24 may, for example, becomprised of organic compounds with extensive π-systems. The individualligand envelopes 24 around the individual inorganic particles 22 enterinto a chemical combination with one another, for example, by way ofpolymerization. Because of the small grain size of the inorganicparticles in the nano range, the light-emitting substance 12 has a highdegree of filling (packing density) so that a plurality of chemicalcompounds exists within the substances 12 between the individual ligandenvelopes 24. This offers a great mechanical stability of thelight-emitting substances 12. The inorganic particles 22 are thusstabilized by means of the ligand envelopes 24 within the substance 12.

The first electrode 14 is comprised of a material with low electron workfunction. The electrode 14 may be comprised of a metal, for example,aluminum, or a metal alloy whose electron work function is smaller than4.5 eV. The second electrode 16 is comprised of a material with highelectron work function. The electrode 16 may, for example, be comprisedof an optically transparent material with good electrical conductivity,for example, indium tin oxide ITO, having an electron work function oflarger than 4.5 eV.

The carrier 20 is comprised of an optically transparent substrate. Likethe electrodes 14 and 16 as well as the substrate 12, the carrier 20 isembodied so as to be planar and is flexible.

The system 10 illustrated in FIG. 1 has the following function:

After the voltage source 18 has been connected with the electrodes 14and 16, for example, by means of a circuit element, not shown, a currentflows between the electrodes 14 and 16 across the light-emittingsubstance 12. The electrical conductivity of the light-emittingsubstance 12 results from the high electron conductivity of theinorganic particles 22. The ligand envelopes 24 around the individualinorganic particles 22 form a potential barrier which has a so-calledtunnel conductivity. The conductivity of electrons or holes between theadjacent inorganic particles 22 is identified as tunnel conductivity.The tunnel conductivity is determined considerably by the spacing of theinorganic particles 22 which is predetermined by the thickness of theligand envelope 24. By connecting the positive pole of the voltagesource 18, the second electrode 16 acts as hole-injecting electrode forthe light-emitting substance 12, while the ground-connected electrode 14is connected as electron-injecting electrode. Since the organic ligandenvelope 24 has a high mobility for holes, the tunneling of theelectrodes is made possible by the ligand envelopes 24 so that, as awhole, an electron conductivity of the light-emitting substance 12 isprovided.

The organic ligand envelopes 24 have functional groups which do not haveto be considered here in detail and which, on the one hand, form trapsfor the polymerization of the individual ligand envelopes 24 among oneanother and, on the other hand, form recombination centers for so-calledexit ions. In this manner, a radiating recombination within therecombination centers takes place during the electron conduction throughthe light-emitting substance 12 so that the light-emitting substance 12can radiate light. This radiating recombination within thelight-emitting substance 12 can be radiated toward the outside by meansof the optically transparent electrode 16 and the optically transparentcarrier 20 so that the electroluminescent system 10 shown in FIG. 1 canserve as a luminous source.

The composition of the light-emitting substance 12 mentioned in theembodiment is merely exemplary. As a whole, the material selection ofthe inorganic particles 12 and of the organic ligand envelope 24 must betailored such that a high electron conductivity is provided in theinorganic particles 22 and a high hole conductivity in the ligandenvelope 24. On the other hand, a high hole conductivity may also bepresent within the inorganic particles 22 and a high electronconductivity in the organic ligand envelope 24. The organic ligandenvelope 24 possesses a high probability for a radiating recombinationof electrons and holes so that this results in a high quantum efficiencyduring electron luminescence of the entire system 10. Furthermore, amechanical, thermal and chemical stability of the light-emittingsubstance 12 is accomplished by way of the functional division among theinorganic particles 22 and the ligand envelopes 24. The two-wayinfluencing of the inorganic particles 22 and of the ligand envelopes 24has the effect that the electron conduction of the light-emittingsubstance 12 is carried by the inorganic particles 22, while the organicligand envelopes are only in an excited state immediately prior to therecombination. This results in a considerably increased long-timestability of the organic ligand envelopes 24 so that a thermal andchemical stability (particularly oxidation) is provided for a long timeperiod.

The system 10 illustrated in FIG. 1 is therefore eminently suited as aluminous source. Owing to the planar production of the system 10, anydesired luminous source may, in principle, be formed from the system 10by means of mechanical working methods, for example, stamping, cutting,etc.. By way of the selection of the organic compounds in the ligandenvelopes, the color spectrum of the light-emitting substance 12 can beset.

FIG. 2 illustrates a further embodiment variant of an electroluminescentsystem 10. Elements that are identical to those in FIG. 1 are providedwith identical reference numerals and are not explained again. Inaddition to the arrangement of the inorganic particles 22 and the ligandenvelope 24 within the light-emitting substance 12 illustrated in FIG.1, molecule chains 28 are arranged here between the ligand envelopes 24.The molecule chains 28 between the individual ligand envelopes 24 thusform a cross linkage for the inorganic particles 22 within thelight-emitting substance 12. As shown in greater detail in FIG. 3, themolecule chains 28 may be, for example, aromatic diamines (upperillustration) or triphenylaminedimers (lower illustration). Here, themolecule chains 28 may have independent functional substituents. Alength of the molecule chains 28 can be set by a number of the atomscomprised in each of the molecule chains 28. Thus, the spacing of theinorganic particles 22 with respect to one another can be set via thenumber of the atoms within the molecule chains 28. The tunnelconductivity between the inorganic particles 22 can be influenced by wayof the variability of the spacing between the inorganic particles 22 inthat the tunnel probability of the electrons can be influenced by theligand envelope 24. As a whole, the light-emitting substance 12 can thusbe optimized to provide a light efficiency which is as high as possible.

By way of the embodiments it becomes clear that it is possible in asimple manner to create large-surface, electroluminescent systems 10 ina cost-advantageous manner which, apart from a high light efficiency,have a high stability from a mechanical, thermal as well as a chemicalviewpoint.

What is claimed is:
 1. An electroluminescent system, comprising:a lightemitting substance which emits light when an electric voltage is appliedthereto and which comprises: inorganic particles which are clustered toprovide inorganic clusters; and organic ligand envelopes which arecomprised of an organic ligand material and which each envelope aninorganic cluster whereby the inorganic clusters are stabilized.
 2. Theelectroluminescent system according to claim 1, wherein the inorganicclusters are one of clusters containing atoms of semiconductingmaterials or clusters containing metal atoms.
 3. The electroluminescentsystem according to claim 1, wherein the organic ligand envelopecomprises functional groups for which electroluminescence is produced bya radiant recombination of holes and electrons.
 4. Theelectroluminescent system according to claim 1, wherein the organicligand envelopes are comprised of organic compounds having an extensiveπ-system.
 5. The electroluminescent system according to claim 4, whereinthe organic compounds chemically combine with one another bypolymerization.
 6. The electroluminescent system according to claim 1,wherein the light emitting substance further comprises organic moleculechains which connect organic ligand envelopes to one another.
 7. Theelectroluminescent system according to claim 6, wherein the organicmolecule chains comprise functional, independent substituents.
 8. Theelectroluminescent system according to claim 7, wherein the inorganicclusters are spaced apart by a spacing which may be preselected byproviding organic molecule chains having a preselected number of atomsin the organic molecule chains.
 9. The electroluminescent systemaccording to claim 1, further comprising first and second electrodesbetween which the light-emitting substance is arranged so as to besubstantially planar and so as to collectively form a layer system. 10.The electroluminescent system according to claim 9, wherein the firstelectrode is comprised of a material having an electron work functionwhich is low and is less than 4.5 Ev.
 11. The electroluminescent systemaccording to claim 10, wherein the second electrode is comprised of amaterial having an electron work function which is high and is greaterthan 4.5 Ev.
 12. The electroluminescent system according to claim 11,wherein at least one of the first and the second electrodes is comprisedof an optically transparent material.
 13. The electroluminescent systemaccording to claim 11, further comprising a carrier on which the layersystem comprised of the first and second electrodes and thelight-emitting substance is arranged.
 14. The electroluminescent systemaccording to claim 13, wherein the carrier is comprised of an opticallytransparent material.
 15. The electroluminescent system according toclaim 14, wherein the carrier is flexible.
 16. The electroluminescentsystem according to claim 1, wherein inorganic clusters and the organicligand envelopes form inorganic cluster compounds.
 17. Theelectroluminescent system according to claim 1, wherein the lightemitting substance further comprises organic spacer molecules whichconnect the organic ligand envelopes to one another.